In a hollow strain wave gearing having a cup-shaped flexible externally toothed gear, a wave generator is arranged on the inner side of the cup-shaped externally toothed gear and a hollow part extends to pass through a boss of the externally toothed gear and a plug of the wave generator. Since the boss and the plug both arranged inside the externally toothed gear are positioned apart from each other in the direction of the center axis line, lubricant may pass through between these parts and leak into the hollow part from the side of the externally toothed gear. It is therefore necessary to seal between these parts. When it is desired to maintain a large hollow diameter, the seal structure is not able to assembled in the hollow part.
Patent Documents 1 and 2 propose, in a hollow actuator having a hollow strain wave gearing and a hollow motor, a lubricant seal structure for preventing lubricant from leaking into the hollow part and for preventing the leaked lubricant from entering the hollow motor side. The lubricant seal structure disclosed in these Patent Documents is provided with a labyrinth seal. The labyrinth seal is formed between the internal circumferential surface of the plug of the wave generator or the internal circumferential surface of the hollow input shaft integrally formed on the external circumferential surface thereof with the plug, and a sleeve (hollow shaft) extending to pass through the plug or the hollow input shaft.